Many steam turbine rotors are fabricated by welding two or more separate forgings together since forgings of some alloys are much better suited to high temperature environments, while forgings of other alloys are best suited for lower temperature environments, and the steam turbine rotor typically has to traverse both high temperature and low temperature environments. Typical techniques for joining such forgings together are disclosed in U.S. pat. Nos. 4,962,586, 5,414,929, and 5,532,454, the disclosures of which are hereby incorporated by reference. In one technique, the forgings are provided with a central, axial cavity or hole at a planned site of a circumferential weld to reduce the depth of the required weld to the minimum necessary for adequate strength. That is, the weld cross section is typically an annulus when viewed axially down a rotor. For some rotor designs, such as impulse turbine designs, the required strength of the minimal depth weld joint thus produced might require an increase in rotor diameter at the welded site, which can be undesirable due to increased annulus area of the packings or steam seals associated with the turbine rotor in use. Additionally, this type of fabrication requires specialized and expensive welding stations capable of handling a rotor vertically, and to carefully make root welds, which can remain in the as-welded final configuration. If less expensive welding stations for horizontally positioned rotors are utilized, they may produce weld root cracking caused by alternating deflections as the material is rotated.
According to the present invention, a steam turbine rotor is provided, and a method of manufacture thereof, which are advantageous compared to typical techniques. According to the present invention it is possible to provide a weld substantially through the entire, or at least a majority of, the rotor cross section rather than providing a relatively small annular weld. However, if such a weld were attempted utilizing conventional processes to produce a single weld, a large deep weld would be required which would be time consuming and difficult to produce. However, that can be avoided, according to the present invention, by providing two different welds, produced by different processes that are suitable for the particular welds involved, and which has a number of advantageous attributes and results. For example, a root weld may be provided which is less than optimum. but allows the rotor forgings to be held together for conventional welding in relatively inexpensive horizontal facilities so that an optimum weld may be produced that does not require a significant bulge at the weld, which would cause an undesirable increases in the annulus areas of packings or steam seals.
According to one aspect of the present invention there is provided a steam turbine rotor comprising: A first steam turbine rotor forging of a first alloy, having a first end and a first diameter. A second steam turbine rotor forging of a second alloy significantly different than the first alloy having a second end and a second diameter. The first and second ends contoured so as to provide first and second central portions. respectively, having third and fourth diameters, respectively, less than the first and second diameters, and first and second contoured portions, respectively, radially outward of the first and second central portions, respectively. The first and second central portions joined by a first weld comprising a friction, inertia, or electroslag weld. And the first and second contoured portions joined by a second weld comprising an arc weld. The first weld need not be normalized or heat tempered because it need not provide strength for the final weld holding the forgings together.
The arc weld may comprise a submerged arc weld. The contoured portions may comprise truncated cones, and the central portions may be disc-shaped truncations of the contoured portions. The central portions may have a diameter of about 20-30% of the diameter of the rotors; for example, the central portions may have a diameter of about 8 inches, whereas the rotor diameter may be 32 inches.
The first alloy may be a CrMo alloy, such as a CrMoV alloy, while the second alloy may be an NiCrMo alloy, such as an NiCrMoV alloy. Preferably the first and second diameters are substantially the same, and the third and fourth diameters are substantially the same.
According to another aspect of the present invention there is provided a method of making a steam turbine rotor from first and second steam turbine rotor forgings of different alloys having first and second ends, respectively, comprising: a) Forming the first and second ends so that each comprises a central portion, and a contoured portion extending axially and radially outwardly from the central portion. b) Bringing the central portions of the different alloy first and second steam turbine rotor forgings into aligned operative association with each other. c) While the central portions are in aligned operative association with each other, welding the central portions of the first and second rotor forgings together using a first welding technique so that the first and second rotor forgings stay together and aligned during subsequent handling. And d) after c), welding the contoured portions of the first and second rotor forgings together while positioned substantially horizontally using a second welding technique, different than the first technique, to form an operable composite steam turbine rotor.
For example, c) may be practiced by friction, inertia, or electroslag welding, and b) may be practiced by arc welding, such as submerged arc welding. Desirably, the central portions of the first and second rotor forgings are substantially disc-shaped and have substantially the same diameter, and b) is practiced to place the central portions in face-to-face aligned position. Also, typically the contoured portions are substantially truncated cone-shaped portions of substantially the same diameter and conic angle, and define an open volume between them, and preferably d) is practiced to substantially fill the open volume by arc welding. Desirably d) is practiced after c) without any intervening procedures for normalizing or heat treating the weld formed by c).
The method also typically further comprises using the turbine rotor so created so that the first rotor forging is disposed within a high temperature area, and the second rotor forging is disposed within a low temperature area, in a steam turbine environment.
According to yet another aspect of the present invention there is provided a steam turbine rotor comprising: A first steam turbine rotor forging of a first alloy, having a first end and a first diameter. A second steam turbine rotor forging of a second alloy significantly different than the first alloy having a second end and a second diameter. The first and second ends contoured so as to provide first and second central portions, respectively, having third and fourth diameters, respectively, less than the first and second diameters, and first and second contoured portions, respectively, radially outward of the first and second central portions, respectively. The contoured portions comprising truncated cones and the central portions comprise disc shaped truncations of the conical contoured portions, and have a diameter about 20-30% of the diameter of the rotors. And a first weld joining the first and second central portions, and a second weld, different than the first weld, joining the first and second contoured portions.
Preferably the first and second diameters are substantially the same, and the third and fourth diameters are substantially the same. The first weld need not be normalized or heat treated. The first alloy may be a CrMoV alloy, and the second alloy may be an NiCrMoV alloy. Also desirably the steam turbine rotor is mounted for rotation, and one of the rotor forgings disposed in a high temperature area, and the other of the rotor forgings disposed in a low temperature area, in a steam turbine environment.